The present invention relates to processes for depositing protective coatings. More particularly, this invention relates to a process for forming an improved bond coat of a thermal barrier coating system.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide turbine, combustor and augmentor components with an environmental coating that inhibits oxidation and hot corrosion, or a thermal barrier coating (TBC) system that thermally insulates the component surface from its operating environment. TBC systems typically include a ceramic layer (TBC) adhered to the component with a metallic bond coat that also inhibits oxidation and hot corrosion of the component surface.
A bond coat is beneficial to the service life of the thermal barrier coating system in which it is employed, and is therefore also beneficial to the service life of the component protected by the coating system. During exposure to the oxidizing conditions within a gas turbine engine, bond coats inherently continue to oxidize over time at elevated temperatures, which gradually deplete aluminum from the bond coat and increases the thickness of the oxide scale. Eventually, the scale reaches a critical thickness that leads to spallation of the ceramic layer at the interface between the bond coat and the oxide scale. Once spallation has occurred, the component will deteriorate rapidly, and therefore must be refurbished or scrapped at considerable cost. In view of the above, there is a continuous need to improve the spallation resistance of such thermal barrier coatings through improvements in the bond coat.